Lipid transfer proteins (LTPs) are low molecular weight proteins that were previously thought to play an important physiological role in transferring lipids between membranes in vitro. The proteins have been characterized in many plant species and are found in a variety of tissues and developmental stages1. They form a multigenic family and more than 50 amino acid sequences of plant LTPs are registered in the genome data banks. Two main families with different molecular masses have been isolated. One is composed by proteins with molecular mass of about 9 kDa and the other, by proteins with molecular mass of 7 kDa, referred to as LTP1 and LTP2, respectively. The LTP1 proteins are basic, presenting isoelectric points (pI) of between 9 and 10. Among the known sequences of LTP 1, all are characterized by having 90 to 95 amino acid residues, of which eight are cysteines conserved in similar positions along the primary structure. These cysteine residues are involved in intramolecular disulphide bridges that have been strictly conserved among LTP1s1. Furthermore, LTPs do not contain aromatic tryptophan or phenylalanine residues. Two well-conserved tyrosine residues are located toward the N- and C-termini of the polypeptide backbone. Proteins in both families are synthesized as precursor proteins and enter into the secretory pathway following a signal peptide cleavage. LTP1s from various plant species are localized at the cell wall in Arabidopsis thaliana2, Zea mays3, Ricinus communis4, and Vigna unguiculata5,6 seeds.
The functional role of LTPs in plants has been extensively debated. In R. communis kernels, a LTP isoform has been found inside an organelle, which was characterized as the glyoxosome. This LTP was shown to increase the activity of the acetyl-CoA oxidase enzyme in in vitro tests, suggesting involvement in βoxidation, possibly in the regulation of the catabolism of lipid storage6. In Brassica oleracea var. italica, LTP was found associated with the waxy surface of the leaves. The expression pattern suggests a role of the LTP in the transport of monomers of cutin4. In addition, abiotic stress factors such as drought, cold, and salt, have been described to upregulate members of the LTP family in some plant species1,7-9. Stabilization of membranes, cuticle deposition and/or changes in cell wall organization have been claimed as their putative roles in the responses to these stress factors7,9,10. In addition, LTPs have a potential role in plant growth and development, including embryogenesis1, germination11, and pollen-pistil interaction12. While the role of LTPs still remains obscure, the role in plant defense mechanisms against phytopathogens such as bacteria, fungi and viruses seems to be well established1,13,14. This has led to the classification of LTPs as pathogenesis-related (PR) proteins, which are included in the PR-14 family14.
Furthermore, LTPs have recently been identified as plant food allergens. They have been identified as complete food allergens, in that they are capable of sensitizing, i.e., inducing specific IgE, as well as eliciting severe symptoms. LTPs appear to be a strong food allergen that are resistant to proteolytic attack and food processing. Stability allows the allergen to reach the gastrointestinal immune system in an immunogenic and allergenic conformation, allowing sensitization and induction of systemic symptoms. LTPs have been reported in fruits of Rosaceae15,16,17 and Vitaceae18 as well as in other plant species such as Aspargus officinalis and B. oleracea var. capitata19,20. Recently, a comprehensive study on maize allergens was conducted by Pastorello's group21. LTP was confirmed to be the major maize allergen by screening sera from 22 patients with systemic symptoms after maize ingestion with 19 (86%) of the patients recognizing the LTP 9 kDa protein. In a follow-up study, LTP was found to be an extremely stable protein, and maintains IgE-binding activity even after cooking at 100° C.22. In addition, maize LTP appears to also be resistant to gastrointestinal digestion23. Collectively, these properties enable members of the LTP class of proteins to be a strong food allergen that can cause severe reactions. Interestingly, maize LTP has been found to be a relevant allergen only in Southern Europe and also in a small group of patients from the U.S., suggesting that sensitization to LTP is relatively uncommon24. It is widely known that eight foods account for over 90% of food allergies, including peanuts, tree nuts, wheat, milk, eggs, crustaceans, soybean, and fish.
The role of LTP is, in many instances, still obscure because accurate absolute quantitation of the protein is difficult. Many of the previous studies are challenged by extensive sample preparation or inadequate, nonsensitive, and nonspecific in vitro bioassays. The commonly employed analytical methods for this purpose are based on immunological approaches. Although immunochemical methods generally are highly sensitive and compatible with high throughput, they suffer from limited specificity. Moreover, the development of antibodies for the target protein is a time-consuming process.